I work with E. coli and P. aeruginosa for my research and I often incubate plates overnight. I sometimes forget to take the plates out after a day or two and find myself wondering if I should use one of the colonies to incubate another plate for further work like plasmid extractions. I was told that I shouldn't use colonies from a plate that have been incubating for longer than a day because the possibility of mutant colonies. How valid is this concern? I.e. what is the chance that a colony selected from a plate that has been incubating for a few days is different in terms of genetic material than the bacteria that I originally inoculated with?

But in general: mutations do occur and the older your sample, the more mutations would have taken place...Another problem might be the total loss of the plasmid altogether.

It also depends on what you are going to do with the bacteria/plasmids.But in general its best to work with fresh material.

There is one famous study about this, however I can remember the name of the one doing it.. Its someone that has been replating bacteria for ages (really a long time) to prove how mutations/resistance etc works...

PS. another problem you are facing is that perhaps you just kept the plates too long at that temperature. Its not good to keep them that long at 37°C.(well also depends on what you want etc..)

If you don't know it, then ask it! Better to ask and look foolish to some than not ask and stay stupid.

There is one famous study about this, however I can remember the name of the one doing it.. Its someone that has been replating bacteria for ages (really a long time) to prove how mutations/resistance etc works...

Are you by chance thinking of Richard Lenski's study on bacterial evolution?

Above all things, if kindness is your king,
then heaven will be yours, before you meet your end

There is one famous study about this, however I can remember the name of the one doing it.. Its someone that has been replating bacteria for ages (really a long time) to prove how mutations/resistance etc works...

Are you by chance thinking of Richard Lenski's study on bacterial evolution?

Yes, thats the one.Could not remember the name.

If you don't know it, then ask it! Better to ask and look foolish to some than not ask and stay stupid.